The invention relates to a low-pressure mercury vapour discharge lamp provided with a radiation-transmitting discharge vessel which encloses a discharge space containing mercury and rare gas in a gastight manner, while also mercury is present in a vapour pressure control member which is in communication with the discharge space during nominal operation, the discharge lamp comprising means for maintaining a discharge in the discharge space.
Such a lamp, referred to as vapour pressure controlled Lamp hereinafter, is known from DE-PS 10 86 804 (1960). The term "nominal operation" in the present description and claims is used for indicating operating conditions under which the mercury vapour pressure is such that the radiation output of the lamp is at least 80% of the output during optimum operation, i.e. under operating conditions where the mercury vapour pressure is ideal. The vapour pressure control member, an amalgam in the known lamp, limits the mercury vapour pressure in the discharge vessel. This renders nominal operation of the lamp possible at comparatively high lamp temperatures such as may occur in the case of a high lamp load, or when the lamp is used in a closed or badly ventilated luminaire.
The mercury vapour pressure at room temperature is comparatively low in the known lamp. The known lamp as a result has the drawback that, when it is operated on a conventional lamp supply, the initial radiation output is also comparatively low, which output is defined here as the radiation output one second after switching-on of the lamp. The run-up period, i.e. the time which the lamp requires for achieving a radiation output of 80% compared with optimum operation, is comparatively long in addition because the vapour pressure rises only slowly after switching-on of the lamp.
A vapour pressure controlled lamp is disclosed in the U.S. Pat. No. 3,227,907 (1966), NL 151 212 (1967), and DE-AS 12 74 228 (1968) in which an electrode ring around an electrode is provided with an auxiliary amalgam in addition to a main amalgam which acts as a vapour pressure control member. Provided the auxiliary amalgam contains sufficient mercury, the lamp will have a comparatively short run-up period. Upon switching-on of the lamp, in fact, the auxiliary amalgam is heated by the electrode, so that it evolves a substantial portion of the mercury present therein comparatively quickly. A condition is that the lamp must have been out of operation sufficiently long before switching-on, so that the auxiliary amalgam has been able to take up sufficient mercury. If the lamp has been out of operation for a comparatively short period, the shortening effect on the run-up period is only weak. In addition, the initial radiation output is even lower then compared with that of a lamp with a main amalgam only because the auxiliary amalgam continues drawing mercury vapour from the discharge space up to the moment of switching-on of the lamp, thus keeping the vapour pressure low. Furthermore, the drawback arises in comparatively long lamps that comparatively much time is required before the mercury evolved by the auxiliary amalgam has spread over the entire discharge vessel, so that such lamps show a comparatively bright zone near the auxiliary amalgam and a comparatively dark zone remote from the auxiliary amalgam for a few minutes after switching-on.
In vapour pressure controlled lamps with an auxiliary amalgam, the quantity of mercury contained in the auxiliary amalgam depends on the quantity of auxiliary amalgam and the time the lamp has been out of operation, called off-time hereinafter. When the auxiliary amalgam in a lamp has a comparatively great mass, the auxiliary amalgam contains so much mercury after a long off-time that a renewed switching-on of the lamp evolves an excess quantity of mercury in the discharge space. It takes a few tens of minutes then before the main amalgam has absorbed this excess quantity. During this time, the mercury vapour pressure is too high and the lamp accordingly has a comparatively low radiation output. When the auxiliary amalgam in a lamp has a comparatively small mass, the auxiliary amalgam on the contrary contains too little mercury after a short off-time for compensating the absorption by the main amalgam during the first tens of minutes after switching-on. This leads to the effect that the mercury vapour pressure rises initially, then falls, and does not rise again until the main amalgam has also assumed its operational temperature. To counteract this effect, U.S. Pat. No. 3,629,641 (1971) proposes the use of a second auxiliary amalgam which is further removed from the electrode and which takes over the role of the first auxiliary amalgam when the latter is incapable of supplying any more mercury.
After switching-off of the lamp, however, it takes a long time before the auxiliary amalgam(s) has (have) absorbed the necessary quantity of mercury again. When this lamp is switched on before that time, the lamp has a long run-up period in spite of the presence of the auxiliary amalgam.